How to Cap a Bay Window

Updated February 21, 2017

The cap or roof of a bay window works like a hat to keep out moisture from snow and rain and helps to prevent heat loss from outside elements of nature. Over time, strong winds can blow away the shingles or a heavy snow can cause the roof cap to sag and collapse or rain can create a leak that requires the repair or replacement of the roof cap. Replacing or repairing a bay window roof cap requires some basic math skills and heavy lifting, but is not a difficult task.

Use workers to lift the roof cap into place.

Nail the cap securely into place.

Insulate the inner cavity of the roof cap with fibreglass filling.

Precisely calculate and cut three plywood sheets to cover the roof cap beams. The middle board is rectangle-shaped, while the two side panels are triangle-shaped.

Use the nail gun to attach the panels to the joists of the roof cap.

Calculate and bevel-cut the exterior trim so that the ends overlap when connected at different positions along the facade of the roof cap.

Prevent moisture from seeping in the roof cap by fastening a metal drip cap down the cap's edge.

Vertically place roofing felt paper on top of the triangle plywood sections. Seal the seam where the felt connects to the house by covering it with a piece of flashing tape.

Cover the middle rectangle section with roofing felt paper. Make sure the felt edges overlap with the triangle edges. Seal all edges by overlapping with flashing tape.

Fasten a double layer of asphalt shingles to the bottom edge of the first row only. The first layer should be head to toe, gritty-side up. The second layer lies flat over the first layer, but in the opposite direction with the smoother side up.

Nail metal flashing at the end of each row, on both sides, where the shingles and the roof cap intersect.

Start with the third row to stagger the shingles so they don't line up from row to row. When needed, cut the shingles to fit.

Attach all the shingles, and then insert a long piece of flashing to the rectangle-shaped section.

Trim and fit the preset wooden shingles. Nail them on top of the flashing to conceal the rough edges of the roof cap.

Things You'll Need

  • 2-by-4 lumber (for roof cap frame)
  • 1-by-4 lumber (for exterior trim)
  • 3 or more workers for heavy lifting
  • Pneumatic nail gun
  • Fibreglass insulation
  • Exterior grade plywood/decking panels
  • Mitre saw
  • Metal drip cap
  • Roofing felt paper
  • Flashing tape
  • Asphalt shingles
  • Hammer
  • Nails
  • Preset wooden shingles


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About the Author

A journalist with more than 25 years of writing experience, Melissa Smith is a contributor to a variety of websites, specializing in an array of topics. She has a Bachelor of Arts in mass communication from the University of North Carolina and a Master of Library Science from North Carolina Central University.